It is well known to provide auxiliary heater systems for water-cooled engines. Such auxiliary heater systems have proved to be particularly useful with water-cooled gasoline and diesel engines for trucks and other large vehicles, as the auxiliary heater can be used with conventional engine coolant to heat the vehicle as well as the vehicle engine. Thus, in extremely cold conditions, it is not necessary to leave a truck engine idling to warm the cab or to keep the engine warm for quick starting. With diesel engines, systems of this type are most useful, for the engine may prove practically impossible to start without an auxiliary heating source in below zero temperatures.
Many known auxiliary heating units for water-cooled vehicle engines circulate some of the cooling fluid from the engine through an auxiliary heater and a fluid to air heat exchanger for the interior of the vehicle before passing the heated fluid back to the vehicle engine. Systems of this type, although ultimately effective, must supply sufficient heated fluid to heat both the engine and the heat exchanger, as they provide no means for bypassing the engine. Thus, the fluid to be heated by the auxiliary heater and supplied to the heat exchanger is cooled by the engine mass, and is therefore much colder than the fluid discharged directly from the heat exchanger. Systems of this type require either extended operation of the auxiliary heater or a large capacity auxiliary heater is sufficient heat is to be rapidly provided by the heat exchanger to the interior of the vehicle in cold conditions. Extended operation of the auxiliary heater requires an extended operation of an electric blower for the heat exchanger which depletes the reserve of the vehicle battery when the engine is not running. Auxiliary heating systems of this type are illustrated by the Kofink et al., U.S. Pat. No. 4,010,895 and the Stolz U.S. Pat. No. 3,795,234.
To eliminate the cooling effect of an inoperative engine, auxiliary heating systems have been developed which are capable of shunting heated fluid around the engine and directly to a heat exchange system for a vehicle cab. A system of this type is illustrated by the Moran U.S. Pat. No. 3,758,031. Although heating systems which bypass the vehicle engine operate more effectively to rapidly heat the vehicle cab during periods when the vehicle engine is shut down, such systems in the past have required an inordinate number of separate valves to accomplish the bypass operation. In such systems, the multiplicity of separate valves employed are flow control valves which cannot be operated with the vehicle engine running or with the vehicle in motion.